Over the past two decades, the popularity and availability of cellular telephones and other telecommunication devices such as pagers has grown dramatically. Cellular networks consist of multiple cells which receive and transmit radio waves to cellular telephones. The geographic area of each cell is served by a cell site, which is comprised of antennas, radio equipment and transmission equipment that allows the cell site to operate with the cellular network.
The original cellular networks were established using omnidirectional antennas of a high gain, allowing a broad area of coverage by each cell site. These cells which cover large geographic areas are typically termed macrocells. A macrocell contains a limited number of radio channels, which limits the amount of traffic the macrocell can process at any given moment. Neighboring macrocells use separate radio channels to prevent co-channel interference problems. To enhance capacity, radio channels are reused at cell sites distant to each other. This spatial separation reduces co-channel interference problems. However, as demand for cellular communications increases, the capacity of macrocells is exceeded, especially in highly populated, urban areas.
To expand cellular capacity, a method of locally reusing cellular radio channels is needed. To accommodate this, cellular networks have added low-power, more localized microcells to the system of powerful macrocells. Microcells can be characterized by their low antenna height, low transmitter power and small coverage area. Directional microcell antennas enhance localized coverage and capacity by radiating radio-frequency (RF) energy into a small, defined area.
A particularly difficult area to maintain coverage is the area located directly away from the corner of a building, and especially when there are a group of buildings as in large downtown or urban areas. This is due to the irregular shape of the area to be covered, which is typically an intersection of two streets. One type of prior art antenna that has been used in such circumstances is a directional panel antenna. Such panel antennas are typically mounted parallel to the sides of buildings. A panel antenna has a solid backplane with an enclosing radome, making it ideal for use close to street level on the sides of buildings where aesthetics prohibit the use of uncovered, screen backplanes. Because most intersections are located at the corner of buildings in urban areas, the resulting radiation pattern from a single antenna mounted parallel to one side of the building does not extend around the building corner to cover the entire intersection.
In an effort to achieve full coverage of an intersection, prior art systems have included two separate panel antennas, each installed on an adjacent side of the building near the intersection. The two antennas are interconnected to a base station with the use of a power combining network, in a configuration that is typically known as co-phasing. This installation requires two panel antennas per intersection.
Requiring multiple antennas not only increases costs and aesthetic impact, but even with several antennas, the intersection is not completely covered. The radiation pattern of two co-phased panel antennas creates a null signal area in the center of the intersection along with destructive interference nulls at other points in the pattern. When a cellular telephone user enters one of the null signal areas in the intersection, transmission and reception ability is degraded. Thus, what is needed is an antenna configuration that provides a smooth and consistent radiation pattern throughout the intersection that avoids this degradation in reception, is aesthetically acceptable, and is cost effective as compared to prior art systems.